1. Technical Field
The present invention relates to MFS type field effect transistors, methods for manufacturing the same, ferroelectric memories and semiconductor devices.
2. Related Art
In recent years, research and development of ferroelectric layers such as PZT, SBT and the like, and ferroelectric memory devices using the same are extensively conducted. Above all, ferroelectric memories using MFS type field effect transistors (1T type) can achieve higher integration, and can readily perform non-destructive reading. The principle of the 1T type ferroelectric memory has been advocated for some tens of years, but is still far from practical use though various challenges have been made so far.
In a MFS type field-effect transistor, a ferroelectric layer is used for a gate insulation layer. However, it is difficult on the process to clean the interface between the ferroelectric layer and the semiconductor layer, and a SiO2 layer is formed at the interface. The retention loss accompanying the leakage current is large, and the retention characteristic is not enough in such a structure. For this reason, a MFMIS structure in which a metallic middle electrode layer is disposed between a ferroelectric layer and an insulation layer has been proposed. However, a paraelectric capacitor is inserted in series with a ferroelectric capacitor in this structure, and a retention loss of data by a phenomenon that is called depolarization occurs. Methods of using a paraelectric material such as HfO2 and the like with a comparatively high dielectric constant for the insulation layer are currently being examined against such a problem.
Pb (Zr, Ti) O3 (PZT) is mainly used so far as a ferroelectric material. In the case of these materials, compositions in a region where rhombohedral and tetragonal coexist with the Zr/Ti ratio being 52/48 or 40/60 or compositions in the neighborhood thereof are used, and also these materials are used with an element such as La, Sr, Ca or the like being doped. This region is used because the reliability, which is most essential for memory devices, is to be secured. Although a Ti rich tetragonal region originally has a good hysteresis shape, Schottky defects that originate in ionic crystal structure occur in the tetragonal region. As a result, defects in leakage current characteristics or imprint characteristics (so-called degree of deformation of hysteresis) are generated, and thus it is difficult to secure the reliability.
Furthermore, although a tetragonal PZT shows a hysteresis characteristic having a squareness that is suitable for memory usage, its reliability is poor and it has not been put to practical use, because of the following reasons.
First, a PZT tetragonal thin film after crystallization has a tendency that the higher the Ti content rate, the higher the density of leakage current rises. In addition, when a so-called static imprint test is conducted, in which data is written once in a memory in either a positive (+) direction or a negative (−) direction, the memory is retained at 100° C., and the data is read, the data written scarcely remains in 24 hours. This is an intrinsic problem of PZT that is an ionic crystal and Pb and Ti that are constituent elements of the PZT, and this poses the most difficult problem of a PZT tetragonal thin film in which the majority of its constituent elements consists of Pb and Ti. This problem is largely attributable to the fact that the PZT perovskite is an ionic crystal, which is an intrinsic problem of PZT.
FIG. 39 is a table showing main energies involved in the bonds of constituent elements of PZT. It is known that PZT after crystallization includes many oxygen vacancies. In other words, it is predicted from FIG. 39 that Pb-O bonds have the smallest bond energy among the PZT constituent elements, and may be readily break at the time of sintering and heating and at the time of polarization inversions. In other words, O escapes due to the principle of charge neutralization when Pb escapes.
Next, the constituent elements of PZT vibrate and repeatedly collide with one another during sustained heating such as at the time of imprint testing or the like. Because Ti is the lightest element among the PZT constituent elements, it would easily come off by these vibrational collisions during high-temperature retention. Therefore, O escapes due to the principle of charge neutralization when Ti escapes. Since the maximum valence of +2 for Pb and +4 for Ti contribute to bonding, there is no way to maintain charge neutrality other than allowing O to escape. In other words, two negative O ions escape readily for each positive ion of Pb or Ti, such that so-called Schottky defects easily form.
Descriptions are now made as to the mechanism of the generation of leakage currents due to oxygen vacancy in PZT crystals. FIGS. 40(A) to 45(C) are views for describing the generation of leakage currents in oxide crystals having a Brownmillerite type crystal structure expressed by the general formula of ABO2.5. As shown in FIG. 40(A), the Brownmillerite type crystal structure is a crystal structure having an oxygen vacancy, as compared to a perovskite type crystal structure of PZT crystals having the general formula ABO3. As shown in FIG. 40(B), because oxygen ions appear adjacent to positive ions in the Brownmillerite type crystal structure, positive ion defects would be difficult to cause leakage current to increase. However, as shown in FIG. 40(C), oxygen ions link the entire PZT crystal in series, and when the crystal structure becomes a Brownmillerite type crystal structure as the oxygen vacancy increases, leakage currents increase accordingly.
In addition to the generation of leakage currents described above, vacancies of Pb and Ti and the accompanying vacancy of O, which are lattice defects, cause a spatial charge polarization shown in FIG. 41. When that happens, reverse electrical fields due to lattice defects that are created by the electrical fields of ferroelectric polarization can occur, causing a state in which the bias potential is impressed to the PZT crystals, and, as a result, hysteresis shift or depolarization can occur. Moreover, these phenomena are likely to occur quicker as the temperature increases.
The problems described above are intrinsic to PZT and it is considered difficult to solve these problems in pure PZT, such that memory elements made by using tetragonal PZT having adequate characteristics have not been achieved so far.
It is an object of the present invention to provide MFS type field effect transistors having ferroelectric layers that would be difficult to have retention loss due to leakage currents and/or depolarization, and methods for manufacturing the same.
It is an object of the present invention to provide ferroelectric memories and semiconductor devices having MFS type field effect transistors in accordance with the present invention.